Radio transmitter/receiver systems designed and built since the 1950's often include data communications capabilities in addition to or instead of voice/audio communications capabilities. The most common interface for connecting data communication equipment (DCE) like radio transmitters and receivers to data terminal equipment (DTE) such as personal computers is the serial interface, defined by the Electronics Industry Alliance (EIA) standards RS-232, RS-422, RS-530, and others.
Serial interfaces provide a common electrical interface for sending and receiving streams of data as a sequence of voltage levels in which one voltage level indicates a digital one (1) and a different voltage level indicates a digital zero (0). Some serial interfaces, known as synchronous serial interfaces, provide timing information in the form of a separate clock signal by which the transmitter and receiver synchronize the rate at which data flows through the interface. Asynchronous serial interfaces do not have this separate clock signal and rely on oscillators within each device to maintain implicit timing. Serial interfaces may also contain separate control signals to provide status information, enable and disable the flow of data, and/or manipulate the state of the DCE.
Radio transmitter/receiver systems in use today may implement any one of a wide range of different serial standards and configurations, but the most common DTE in use today is the personal computer (PC) which only provides an asynchronous RS-232 interface. As a result, PCs are unable to utilize many radio transmitter/receiver systems that provide a serial interface based on a different standard, and are unable to take advantage of advanced wireless communications protocols that require a synchronous serial interface between the PC and the radio.
To alleviate this incompatibility, specialized hardware in the form of add-on boards is commercially available that augments standard PCs with additional serial interfaces that are compatible with some radios. However, this hardware is only compatible with a limited number of commonly-used radios, and the add-on boards are specific to the form-factor of the PC, depending on whether the PC is intended for a desktop, industrial, or mobile environment.
Interfacing with radio transmitter/receiver systems using serial links also imposes additional restrictions on the DTE. Most radio equipment operates only in a half-duplex mode, in which a radio device is unable to transmit and receive simultaneously. The DTE must control the direction of data exchange by manipulating the “push-to-talk” (PTT) control signal on the serial interface to activate and de-activate the transmitting circuitry on the radio device. Further, many radio transmitters impose additional time delay requirements when transitioning between transmit and receive mode; for example, a transmitter may require several hundred milliseconds for the amplifiers to ramp up to full power before data may be sent, or the data modulator may require a short delay to finish flushing all of the data to be transmitted out of the internal buffers. These timing requirements often necessitate custom software on the DTE that is specific to the serial interface circuitry and/or radio system.
For some uses, particularly in the military environment, a single PC must be able to interface with multiple radios to provide services that translate between or aggregate the communications capabilities of multiple radio links. These radios are often located distant from one another due to antenna positioning requirements, physical space constraints, or safety reasons. Standard serial interfaces typically frustrate the implementation of such multi-radio systems due to the high cost and technical difficulty of extending serial links more than a few feet from the DTE to the DCE. Providing a simple and low-cost means of extending the range of the link between a PC and a radio would increase the potential capabilities of radio data systems.
Prior art exists that solves some, but not all, of these issues in a limited environment for some radio systems. Remote control units for radios are able to extend the serial link between the DTE and the radio for hundreds of feet or even several miles, but these units are specific to a particular model of radio system and the serial interface is still unlikely to be compatible with a standard PC. Converters are available to interface from certain radio-specific serial interfaces to a common PC serial interface, but these converters prevent the use of more efficient data encoding mechanisms and do not provide a means of extending the distance between DTE and radio.